INTRODUCTION:

Superdisintegrant are the agents added to tablet and some encapsulated formulations to promote the breakup of the tablet and capsule “slugs’ into smaller fragments in an aqueous environment there by increasing the available surface area and promoting a more rapid release of the drug substance. They promote moisture penetration and dispersion of the tablet matrix¹. Tablet disintegration has received considerable attention as an essential step in obtaining fast drug release. The emphasis on the availability of drug highlights the importance of the relatively rapid disintegration of a tablet as a criterion for ensuring uninhibited drug dissolution behavior. Number of factors affects the disintegration behavior of tablets. The disintegrants have the major function to oppose the efficiency of the tablet binder and the physical forces that act under compression to form the tablet. The stronger the binder, the more effective must be the disintegrating agents in order for the tablet to release its medication.

Ideally, it should cause the tablet to disrupt, not only into the granules from which it was compressed, but also into powder particles from which the granulation was prepared. Disintegrants are an essential component to tablet formulations². The ability to interact strongly with water is essential to disintegrant function. Combinations of swelling and/or wicking and/or deformation are the mechanisms of disintegrant action. A disintegrant used in granulated formulation processes can be more effective if used both “intragranularly” and “extragranularly” thereby acting to break the tablet up into granules and having the granules further disintegrate to release the drug substance into solution. However, the portion of disintegrant added intragranularly (in wet granulation processes) is usually not as effective as that added extragranularly due to the fact that it is exposed to wetting and drying (as part of the granulation process) which reduces the activity of the disintegrant. Since a compaction process does not involve its exposure to wetting and drying, the disintegrant used intragranularly tends to retain good disintegration activity. There are three methods of incorporating disintegrating agents into the tablet: A. Internal Addition (Intragranular) B.External Addition (Extragranular) C. Partly Internal and External. In a direct compression process, drug is blended with a variety of excipients, subsequently lubricated and directly compressed into a tablet. A disintegrant used in this type of formulation, simply has to break the tablet apart to expose the drug substance for dissolution. Most common tablets are those intended to be swallowed whole and to disintegrate and release their medicaments rapidly in the gastrointestinal tract (GIT). The proper choice of disintegrant and its consistency of performance are of critical importance to the formulation development of such tablets. In more recent years, increasing attention has been paid to formulating not only fast dissolving and/or disintegrating tablets that are swallowed, but also orally disintegrating tablets that are intended to dissolve and/or disintegrate rapidly in the mouth. Most prior studies have focused on the functionally related properties of Superdisintegrants with special emphasis on correlating these functional properties to disintegrant efficiency and drug release rate. Water penetration rate and rate of disintegration force development are generally positively related to disintegrant efficiency in nonsoluble matrices. However, such a positive correlation is not always observed between tablet disintegration time and drug dissolution rate^{1, 2, 3}.

Mechanism action of disintegrants:

· By capillary action

· By swelling

· Because of heat of wetting

· Due to release of gases

· By enzymatic action

· Due to disintegrating particle/particle repulsive forces

· Due to deformation

Fig1: Disintegration of tablet by swelling

· Capillary action: Disintegration by capillary action is always the first step. When we put the tablet into suitable aqueous medium, the medium penetrates into the tablet and replaces the air adsorbed on the particles, which weakens the intermolecular bond and breaks the tablet into fine particles. Water uptake by tablet depends upon hydrophilicity of the drug /excipient and on tableting conditions. For these types of disintegrants, maintenance of porous structure and low interfacial tension towards aqueous fluid is necessary which helps in disintegration by creating a hydrophilic network around the drug particles⁴.

· Swelling: Perhaps the most widely accepted general mechanism of action for tablet disintegration is swelling Tablets with high porosity show poor disintegration due to lack of adequate swelling force. On the other hand, sufficient swelling force is exerted in the tablet with low porosity. It is worthwhile to note that if the packing fraction is very high, fluid is unable to penetrate in the tablet and disintegration is again slows down⁴.

· Heat wetting: When disintegrants with exothermic properties gets wetted, localized stress is generated due to capillary air expansion, which helps in disintegration of tablet. This explanation, however, is limited to only a few types of disintegrants and cannot describe the action of most modern disintegrating agents⁴.

· Due to release of gases: Carbon dioxide released within tablets on wetting due to interaction between bicarbonate and carbonate with citric acid or tartaric acid. The tablet disintegrates due to generation of pressure within the tablet. This effervescent mixture is used when pharmacist needs to formulate very rapidly dissolving tablets or fast disintegrating tablet. As these disintegrants are highly sensitive to small changes in humidity level and temperature, strict control of environment is required during manufacturing of the tablets. The effervescent blend is either added immediately prior to compression or can be added in to two separate fraction of formulation⁴.

· By enzymatic action: Here, enzymes present in the body act as disintegrants. These enzymes destroy the binding action of binder and helps in disintegration. Actually due to swelling, pressure exerted in the outer direction or radial direction, it causes tablet to burst or the accelerated absorption of water leading to an enormous increase in the volume of granules to promote disintegration⁴.

Fig2: Disintegration of tablet by deformation

· Due to deformation: Hess had proved that during tablet compression, disintegrated particles get deformed and these deformed particles get into their normal structure when they come in contact with aqueous media or water. Occasionally, the swelling capacity of starch was improved when granules were extensively deformed during compression^{4, 5}.

· Disintegration of tablet by repulsion: Another mechanism of disintegration attempts to explain the swelling of tablet made with ‘non‐swellable’ disintegrants. Guyot‐ Hermann has proposed a particle repulsion theory based on the observation that no swelling particle also causes disintegration of tablets. The electric repulsive forces between particles are the mechanism of disintegration and water is required for it. Researchers found that repulsion is secondary to wicking^{4, 5}.

Fig3: Disintegration of tablet by repulsion

Methods of Incorporating Disintegrants into Tablets^7,8,9:

There are two methods of incorporating disintegrating agents into the tablet as described below

· Internal Addition (Intragranular): In Internal addition method, the disintegrant is mixed with other powders before wetting the powder mixtures with the granulating fluid. Thus the disintegrant is incorporated within the granules.

· External Addition (Extragranular) : In external addition method, the disintegrant is added to the sized granulation with mixing prior to compression.

· Partly Internal and External: In this method, part of disintegrant can be added internally and part externally. This results in immediate disruption of the tablet into previously compressed granules while the disintegrating agent within the granules produces additional erosion of the granules to the original powder particles.

Various Available Superdisintegrant from Different Sources^8-17:

· Modified starch (sodium starch glycolate): It is possible to synthesize sodium starch glycolate from a wide range of native starches, but in practice potato starch is used as it gives the product with the best disintegrating properties. After selection of the appropriate starch source the second step is the cross linking of the potato starch. This is typically carried out using an FDA approved starch esterifying agent such as sodium trimetaphosphate or phosphorus oxychloride in alkaline suspension. The effect of introduction of the large hydrophilic carboxymethyl groups is to disrupt the hydrogen bonding within the polymer structure. This allows water to penetrate the molecule and the polymer becomes cold water soluble. The effect of the cross linking is to reduce both the water soluble fraction of the polymer and the viscosity of dispersion in water. The optimum balance between the degree of substitution and the extent of cross-linking allows for rapid water uptake by the polymer without the formation of a viscous gel that might impede dissolution.

· Crosslink polyvinlypyrrolidone (crospovidone): Crospovidone quickly wicks saliva into the tablet to generate the volume expansion and hydrostatic pressures necessary to provide rapid disintegration in the mouth. Unlike other Superdisintegrants, which rely principally on swelling for disintegration, Crospovidone Superdisintegrants use a combination of swelling and wicking? When examined under a scanning electron microscope, crospovidone particles appear granular and highly porous. This unique, porous particle morphology facilitates wicking of liquid into the tablet and particles to generate rapid disintegration. Due to its high crosslink density, crospovidone swells rapidly in water without gelling. Other Superdisintegrants have a lower crosslink density and, as a result, form gels when fully hydrated, particularly at the higher use levels in ODT formulations. Unlike other Superdisintegrants which are either poorlycompressible or non-compressible, Crospovidone disintegrants are highly compressible materials as a result of their unique particle morphology. In contrast to sodium starch glycolate and croscarmellose sodium, Crospovidone Superdisintegrants exhibit virtually no tendency toward gel formation, even at high use levels. Disintegrants that gel can result in ODT and chewable products with an unpleasant, gummy texture. Crospovidone Superdisintegrants provide the best overall sensory experience as well as rapid disintegration and robust tablets.

· Modified cellulose (crosscarmelose sodium): Croscarmellose sodium is described as a cross-linked polymer of carboxymethylcellulose. Apart from the differences between the starch and cellulose polymer backbones, there are Differences between the synthetic processes used to modify the polymer. Most importantly, the DS of croscarmellose sodium is higher than that of sodium starch glycolate, and the mechanism of cross linking is different. The substitution is performed using Williamson’s ether synthesis to give the sodium salt of carboxymethylcellulose. A key difference from the chemistry of SSG is that some of the carboxymethyl groups themselves are used to cross-link the cellulose chains, the process being accomplished by dehydration. Thus the cross-links are carboxyl ester links rather than phosphate ester links as in Primojel.

Modified Resin:

· Ion Exchange Resin: The INDION 414 and KYRON 314 have been used as a superdisintegrant for ODT. It is chemically cross-linked polyacrylic potassium, with a functional group of – COO – and the standard ionic form is K+. It has a high water uptake capacity. It is a high purity pharmaceutical grade weak acid cation exchange resin supplied as a dry powder. It is an extremely effective tablet disintegrant which provides the necessary hardness and chemical stability to the tablet. The product swells up to a very great extend when in contact with water or gastrointestinal fluids causing rapid disintegration without the formation of lumps. It is a high molecular weight polymer, therefore it is not absorbed by the human tissues and totally safe for human consumption.

Mucilage as disintegrants:

· Hibiscus rosa-sinensis Linn. Mucilage: Hibiscus rosa-sinensis Linn of the Malvaceae family is also known as the shoe‐flower plant, China rose, and Chinese hibiscus. The plant is available in India in large quantities and its mucilage has been found to act as a superdisintegrant. The plant contains cyclopropanoids, methyl sterculate, methyl‐2‐hydroxysterculate, 2‐hydroxysterculate malvate and β‐rosasterol. The leaves contain carotene (7.34 mg/100 g of fresh material) moisture, protein, fat, carbohydrate, fibers, calcium, and phosphorus. Mucilage of Hibiscus rosa-sinensis contains L‐rhamnose, D‐galactose, D‐galactouronic acid, and D‐glucuronic acid.

The percentage yield of mucilage is estimated as 17%. Other physicochemical parameters of mucilage are also evaluated. The results of swelling ratio, angle of repose, bulk density and compressibility index are observed as 9, 26.5oC, 0.65g/cc, 16% respectivel^29,30.

· Isapghula Husk Mucilage (Plantago ovata): Isapghula Husk consists of dried seeds of the plant known as plantago ovata. The plant contains mucilage in the epidermis of the seeds. Mucilage of plantago ovata has various characteristics like binding, disintegrating and sustaining properties. Mucilage can be used as superdisintegrant to formulate fast dissolving tablets because it has very high percentage of swelling index (around 89±2.2%v/v) as compared to the other superdisintegrating agents. The rapid disintegration of the FDTs is due to the swelling of Superdisintegrants to create enough hydrodynamic pressure for quick and complete disintegration of the tablet. The rate at which swelling develops and significant force of swelling also determine its disintegrating efficiency^31,33.

· Cucurbita maxima pulp powder: Cucurbita maxima fruit was cleaned with water to remove dust from surface and further peel was removed. The seed was removed and pulp was put into juicer mixer to form highly viscous liquid. This was further lyophilized to get solid porous mass. Size reduction was done and powder was collected. The collected powder was passed through 80 # sieve and stored for further study. Study revealed that Cucurbita maxima pulp powder have comparable dissolution behavior to that of sodium starch glycolate. It also has comparable hardness and friability thus the naturally obtained Cucurbita maxima pulp powder stands as a good candidate to act as disintegrant and it is possible to design promising Fast disintegrating tablet using this polymer³⁵.

· Lepidium sativum Seed Mucilage: Natural Lepidium sativum (family: Cruciferae), also known as asaliyo, has wide application in pharmaceutical field as disintegrating agent and as herbal medicine. Seeds contain a higher proportion of mucilage, dimeric imidazole alkaloids lepidine B, C, D, E and F and two new monomeric imidazole alkaloids semilepidinoside A and B. The mucilage can be extracted from seeds by different procedures and its yield varies from 14% to 22%. Mucilage of Lepidium sativum has various characteristic like binding, disintegrating, gelling etc. The extracted mucilage is used to develop fast dissolving tablets. Mucilage is found to be a brownish white powder which decomposes above 200oC and have characteristic odour. On evaluating its various physicochemical characteristics, the values of swelling index, angle of repose, bulk density and tapped density are estimated as following 18, 32oC, 0.58g/cc and 0.69g/cc respectively³⁶ .

· Fenugreek Seed Mucilage: Trigonella Foenum-graceum (family Leguminosae), commonly known as Fenugreek, is an herbaceous plant of the leguminous family. It is one of the oldest cultivated plants and has found wide applications as a food, a food additive, and as a traditional medicine in every region. Fenugreek seeds contain a high percentage of mucilage which can be used as disintegrant for use in mouth dissolving tablet formulations. Mucilage is an off white-cream yellow coloured amorphous powder that quickly dissolves in warm water to form viscous colloidal solution. Its physicochemical parameters are studied and found to have 22.25oC, 0.64g/cc, 15.20% values as angle of repose, bulk density and compressibility index respectively²⁸.

· Chitosan: Chitosan is a natural polymer obtained by deacetylation of chitin which is the second most abundant polysaccharides in nature after cellulose. Superdisintegrant property of chitosan has been utilized to develop a fast mouth dissolving tablet by utilizing a novel met. Similar to the other Superdisintegrants chitosan too generously engulf water when in contact with aqueous media and burst due to the pressure exerted by their capillary action thereby impart instantaneous disintegration of the dosage form and

resulting in formation of a uniform dispersion in the surrounding media which behave like a true suspension formed inside the body leading to rapid and complete absorption of drug³⁷.

Gum as disintegrant⁴⁰:

Gums: Gums have been used as disintegrants because of their tendency to swell in water. They can perform good disintegration characteristics (2-10% w/w of tablet weight) and the amount of gum must be carefully titrated to determine the optimum level for the tablet. Gums, which are commonly used as disintegrants consist of guar gums, karaya, gellan, agar, pectin and tragacanth.

· Guar Gums: Guar gum is naturally occurring guar seed extract, containing about 80% of galactomannan (guaran), 10% moisture, 5-7% protein and trace amounts of heavy metals and ash. It is free flowing, completely soluble, neutral polymer and is approved for use in food. It is not sensitive to pH, moisture contents or solubility of the tablet matrix. It is not always pure white and sometimes varies in color from off-white to tan tends to discolour with time in alkaline tablets. As a disintegrant, guar gum has been found to be superior to some common disintegrants such as corn starch, celluloses, alginates and magnesium aluminium silicate. Particle size can affect disintegration, with finer particle sizes having greater disintegrating capabilities. It is available in the market under the trade name jaguar.

· Gellan Gums: Gellan gum is a linear anionic polysaccharide, biodegradable polymer produced by the microbe Pseudomonos elodea consisting of a linear tetrasaccharide repeat structure and used as a tablet disintegrant. Gellan polymer consists of monosaccharide α-L-rhamnose, β-D-glucuronic acid and β-D-glucose in molar ratio of 1:1:2 linked together to form a linear primary structure. The disintegration of tablet might be due to the instantaneous swelling characteristics of gellan gum when it comes into contact with water and owing to its high hydrophilic nature. In a study, the complete disintegration of tablet was observed within 4 minutes with gellan gum concentration of 4 % w/w and 90 % of drug dissolved within 23 minutes.

· Gum Karaya: Karaya has the natural gum exudates from the traces of Sterculiaurens belonging to family sterculiacea. Chemically the gum has an anionic polysaccharide, containing 43%. D-galacturonic acid, 13% D-galactose and 15 percent L-rhamnose. It absorbs water and swells to 60-100 times their original volume. The high viscosity nature of gum limits its uses as binder and disintegrant in the development of conventional dosage form.

· Agar: Agar is the dried gelatinous substance obtained from Gelidium amansii (Gelidanceae) and several other species of red algae like, Gracilaria (Gracilariaceae) and Pterocadia (Gelidaceae). Agar is yellowish gray or white to nearly colorless, odorless with mucilaginous taste and is accessible in the form of strips, sheet flakes or coarse powder. Agar consists of two polysaccharides as agarose and agaropectin. Agarose is responsible for gel strength and Agaropectin is responsible for the viscosity of agar solutions. It is a potential candidate to act as a disintegrant due to its high gel strength . Gums are used in concentration from 1 to 10%. However, these are not as good disintegrating agents as others because capacity development is relatively low.

CONCLUSION:

With the increase demand of novel drug delivery, the fast disintegrating drug delivery system has become one of the mile stone of present investigation. The ease of availability of these agents and the simplicity in the direct compression process suggest that their use would be a more economic alternative in the preparation of ODT than the sophisticated and patented techniques.

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Received on 09.01.2012 Accepted on 05.03.2012

Asian J. Pharm. Tech. 2(1): Jan.-Mar. 2012; Page 19-25

Synthetic superdisintegrant	Properties	Effective concentration for disintegrants
Crospovidone	It is completely insoluble in water. Rapidly disperses and swells in water. Greatest rate of swelling compared to other disintegrants. Greater surface area to volume ratio than other disintegrants. Available in micronized grades if needed for improving state of dispersion in the powder blend. Swelling index- 58±1.5% v/v.	It is used in the range of 1-3% w/w.
Croscarmellose sodium	It is insoluble in water, although it rapidly swells to 4-8 times its original volume on contact with water. Specific surface area- 0.81-0.83 m2/g. Swelling index- 65±1.7% v/v.	It may be used as a tablet disintegrant at concentration up to 5% w/w, although normally 2 % w/w is used in tablets prepared by direct compression and 3 % w/w in tablets prepared by wet-granulation process.
Sodium starch glycolate	Absorbs water rapidly, resulting in swelling up to 6%. High concentration causes gelling and loss of disintegration. Swelling index- 52±1.2% v/v.	It is used in the range of 4-6%. Above 8%, disintegration times may actually increase due to gelling and its subsequent viscosity producing effects
Polacrilin Potassium	No lump formation after disintegration. High compatibility with excipients and common therapeutic	Used as a tablet disintegrant and as a taste-masking agent for various drugs.

Mucilage	Drug	Approach Used	Result
Lepidium Sativum	Nimesulide	Direct compression	Disintegration time of 17 sec. and mean dissolution time 5.27 sec. at 10% w/w concentration, found better than other synthetic disintegrants like Ac-di-sol and SSG.
Plantago ovata mucilage	Prochlorperazine maleate	Direct compression	Dispersion time of 8 sec. at concentration of 8 % w/w
Hibiscus rosa-sinensis Linn. mucilage powder	Aceclofenac	Direct compression	At concentration of 6 % w/w showed disintegration time of 20 sec.
Fenugreek seed mucilage	Metformin hydrochloride	Direct compression	It shows 15.6 sec. disintegration time and 100% drug release within 18 min. at concentration of 4 % w/w. while croscarmellose sodium shows disintegration time of 28 sec. at optimum concentration (8%).
Ocimum gratissimum mucilage powder and seed powder	Metformin hydrochloride	Direct compression	Mucilage powder and seed powder both at concentrations of 5 %w/w showed disintegration time of 43 sec. and 45 sec. respectively
Chitosan	Cinnarizine	Wet granulation	Good mouth feel and disintegration time of 60 sec. at the level of 3 % w/w.

Maratha Mandals College of Pharmacy, Belgaum-590016. Karnataka

*Corresponding Author E-mail: shekhar_bio@yahoo.co.in